1. Field of the Invention
The present invention relates to apparatus for measuring the flatness of a surface by scanning the surface with a scanning beam and detecting the angular deviation of the beam of light regularly reflected from the surface.
2. Description of the Prior Art
Various types of flatness measuring apparatus have been known and used. Among them, an interferometer making use of the interference phenomenon of light and an electrostatic capacity type of apparatus making use of 1 change in electrostatic capacity are most widely known. The former is capable of giving two-dimensional information concerning flatness. However, it has a particular disadvantage in that extremely high precision is required for setting the surface to be measured and also that measurement of unevenness and quantative measurements are very difficult to carry out. While the latter mentioned flatness measuring apparatus also allows measurement of the thickness of the subject surface, it has the disadvantage that adjustment of capacity should be made every time the kind of material of the test piece is changed.
In order to eliminate the above described disadvantages, there has been proposed a type of flatness measuring apparatus in which a surface to be measured is scanned by a scanning beam essentially normally incident upon the surface and the angular deviation of the scanning beam of light regularly reflected from the surface is detected in order to determine the flatness of the surface. A typical example of such flatness measuring apparatus is disclosed in U.S. Pat. No. 3,885,875. In the measuring apparatus disclosed therein, the detected angular deviation concerning the inclination of the surface in the direction of scanning, that is, the primary differentiation value of the surface shape relative to scanning time is taken up as the corresponding positional deviation found on a position detecting photoelectric element. An electric integration of the found values gives the value of flatness of the subject surface.
However, this type of flatness measuring apparatus also has the following disadvantage:
When the surface to be measured is a surface on which light is uniformly scattered as in the case of a roughened surface or the like, the measuring apparatus becomes inoperative due to the loss of measuring light. As mentioned above, in the known apparatus, a scanning beam of light is normally incident on the subject surface. Therefore, if the subject surface is a roughened surface having light scattering characteristics, the amount of light regularly reflected from the subject surface toward the collimator lens is reduced to a such extent that no measuring of flatness is possible.
The present invention solves the problem effectively by obliquely projecting a scanning beam on a surface to be measured in a plane normal to the scanning direction so as to decrease the amount of light scattered and instead to increase the amount of light regularly reflected from the surface.